Improved decomposition of DiagonalQubitUnitary

[dwierichs]

Context:
A diagonal unitary operator can be decomposed iteratively into RZ multiplexers (SelectPauliRot) and a global phase.
PennyLane is not using this quite established decomposition yet.
Similar to the QubitUnitary decomposition implemented in #7277, the method implemented here stems from Shende et al. (2004).
It uses fewer CNOTs that the current implementation, but has larger classical computing/memory cost.

Description of the Change:
Replace current decomposition of DiagonalQubitUnitary by multiplexer-based decomposition.

Also fixes a validation in __init__ of SelectPauliRot to allow for parameter broadcasting.

Benefits:
Quantumly cheaper decomposition of DiagonalQubitUnitary.

Possible Drawbacks:
Increase classical computing /memory cost if the decomposition is carried out down to elementary gates (due to SelectPauliRot decomposition).
This increased classical cost is remedied by #7377

Related GitHub Issues:

[sc-90706]

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.65%. Comparing base (e21660f) to head (6d205c7).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7370      +/-   ##
==========================================
- Coverage   99.65%   99.65%   -0.01%     
==========================================
  Files         529      529              
  Lines       50610    50603       -7     
==========================================
- Hits        50434    50427       -7     
  Misses        176      176              

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